1. Field of the Invention
The present invention relates to semiconductor devices, and, more particularly, to transistors having recessed gates and methods of forming the same.
2. Description of the Related Art
Many methods of forming oxide layers have been introduced in recent years. Typical examples of such methods may include thermal oxidation and radical oxidation. Thermal oxidation, which involves heating a substrate while exposing it to an oxygen ambient, has been conventionally employed as a typical method. In radical oxidation, the substrate is exposed to an ambient predominantly made up of oxygen radicals (O*).
When thermal oxidation is applied, the growth rate and thickness of the oxide layer may be greatly influenced by crystal orientation in the substrate. In particular, the growth rate of the oxide layer may be faster in directions having a high area density <110> than in directions having a low area density <100>. In addition, defects may occur in the interface between the oxide layer and the substrate in directions having high a area density. For example, weak bonds and dangling bonds may be formed in the interface between the oxide layer and the substrate in directions that have a high area density. The weak bonds and the dangling bonds may be caused by incompletely oxidized silicon, such as Si2O and Si2O2.
FIGS. 1 and 2 are cross-sectional views illustrating methods of forming oxide layers using thermal oxidation.
FIG. 1 illustrates how growing conditions of the oxide layer can be influenced by crystal orientation in the substrate. Referring to FIG. 1, the substrate 10 having a <100> crystal direction is etched to form a trench 12. The bottom of the trench 12 has a <100> direction, and the sidewalls of the trench 12 have <110> directions.
Referring to FIG. 2, a thermal oxidation process is applied to the substrate 10 to form a thermal oxide layer 14 on a surface of the substrate 10 including the inner walls of the trench 12. As shown in FIG. 2, due to differences in growing rates due to crystal orientation, the thickness (A) of the oxide layer at the bottom of the trench having a <100> direction is thinner than the thickness (B) of the oxide layer on the sidewalls of the trench having <110> directions. In addition, due to stress incurred during the oxide layer formation, the growth of the oxide layer is suppressed at the edges 16 of the trench 12, forming a thinner oxide layer.
As previously mentioned, there may be several problems associated with an oxide layer formed on the sidewalls of a trench 12 having <110> crystal directions. Furthermore, when a strong electric field is applied to the edge of the trench 12, breakdown voltage may be decreased and leakage current may more easily occur at the edge of the trench 12. In addition, in a transistor having a recessed gate (which includes a gate pattern formed in the trench 12 and the thermal oxide layer 14 as a gate oxide layer), devices may operate inadequately if the thermal oxide layer 14 is of low quality.